April 4 marked 50 years since the assassination of Martin Luther King Jr. While we remember him as the leader of the civil rights movement, we forget that his murder came at a time when he was advocating for the rights of unions and the working class.

King was starting to explore a dangerous notion — the unification of races through their class struggle. He went to Memphis, Tennessee, 50 years ago to protest alongside sanitation workers who had been on strike for months. He was starting...